Prior art Metal Oxide Semiconductor Capacitors (MOSCAPs) are formed by depositing a layer of metallic conductor material onto a layer of oxide that has itself been deposited or grown on a layer of semiconductor material. MOSCAPs are often used to provide impedance matching functions in transistor device packages.
In certain applications, particularly when used with high frequency power transistors where high VSWR frequently occurs, reverse bias operation can result. This mode of operation drives the MOSCAP into a deep depletion mode. Whereas in low frequency operation reverse bias operation will typically result in the formation of an inversion layer that serves to limit the depletion layer thickness, high frequency operation causes the depletion region to not stabilize in an inversion layer because of long carrier lifetimes in the semiconductor. As a result, an unacceptably deep depletion region can form, or alternatively, avalanche breakdown can result. As a result of the above, the MOSCAP itself may self destruct and/or cause the associated transistor to become permanently nonfunctional.
Accordingly, there exists a need for a MOSCAP that will function normally during high frequency, reverse bias operating conditions.